Amor, 26, first joined Super League with Leeds back in 2010, from hometown club Whitehaven, and moved to the Wildcats initially on loan, before joining on a permanent basis at the end of 2011.
Saints Head Coach Nathan Brown said: “Kyle is a young British prop forward who is already one of the best front rowers in Super League. He will add strength, size and dynamism to our pack and is a great long term recruit for the club.”
Amor becomes the latest player to leave the troubled Wildcats – half-back Tim Smith was recently sold to Salford for an undisclosed fee.
Wakefield remain determined to retain Dream Team pick Danny Kirmond, despite a six-figure bid from Warrington for the back-rower.